1. Field of the Invention
The invention relates to a polishing slurry for polishing an aluminum film and a polishing method for polishing an aluminum film using the same, and particularly relates to a polishing slurry for polishing an aluminum film used for forming damascene wirings in semiconductor integrated circuits (hereinafter referred to as LSI) and a method for polishing an aluminum film using the same.
2. Description of the Related Art
In recent years, along with high integration and high performance LSI, new techniques for fine patterning have been developed. A chemical mechanical polishing (hereinafter referred to as CMP) process is one of them, and is a frequently used technique in the LSI manufacturing process, particularly in the planarization of interlayer insulating films, formation of metal plugs, and formation of buried wirings (damascene wirings) in the multilayer wiring formation process. The technique is, for example, disclosed in U.S. Pat. No. 4,944,836.
The damascene wiring technique allows to simplify the wiring process and improve yields and reliability, and the application of the technique will be expanded in future. At present, as a wiring metal used for damascene wirings, copper having low resistance is mainly used for high-speed logic devices, and inexpensive aluminum or tungsten is used for memory devices such as DRAM.
However, in either devices, aluminum, which has second lowest resistance following copper, is attracting attention as a wiring metal in consideration of its low resistance and low cost.
In a common method of metal CMP for polishing a metal film, a polishing pad is attached to a circular polishing table (platen), the surface of the polishing pad is moistened with a polishing slurry, the substrate surface having a metal film formed thereon is pressed against the polishing pad, and the polishing table is rotated while a specified pressure (hereinafter referred to as polishing pressure) is applied from the backside of the substrate to remove the metal film in the convex parts through mechanical friction between the polishing slurry and the convex parts of the metal film.
The polishing slurry used in CMP usually comprises an oxidant and abrasive, to which a metal oxide solubilizer is further added as necessary. The basic mechanism of CMP by the polishing slurry for CPM is considered as follows: the surface of a metal film is oxidized by the oxidizer, and the resulting oxide layer is shaven off by the abrasive.
The oxide layer on the metal surface in the concave parts has almost no contact with the polishing pad, so that the abrasive has no shaving effect on that oxide layer. Therefore, along with the progress of CMP, the metal layer on the convex parts is removed and the surface of the substrate is planarized. The details of this mechanism are disclosed in Journal of Electrochemical Society, Vol. 138, No. 11 (issued in 1991), pp. 3460-3464.
In CMP, high-speed polishing of metal films, evenness of the polished surface, and low defect density on the polished surface are required.
Since an aluminum film is softer than other metal films for damascene wirings such as a copper film or tungsten film, it can be polished at a high speed, but when subjected to CMP using a polishing slurry containing a relatively hard polishing abrasive such as alumina particles, among the defects on the polished surface, particularly surface roughness is increased to impair the smoothness and significantly deteriorate the wiring yield.
Thus, to the recent demand for higher quality in LSI, it was difficult with the conventional techniques to polish the surface of an aluminum film to a sufficient smoothness and at a high speed.